Parasitic oscillation modes and Amplified Spontaneous Emission (ASE) losses limit the choices designers have in designing highly optimized optical systems. For example, in high-power laser welders and in many other industrial and military laser applications, the desire is for an optical output with high gain and/or a great amount of power. However, the state-of-the-art optical systems used in these applications have a high potential for parasitic modes and ASE loss, thus requiring design tradeoffs and use of less efficient optical sources.
A basic optical system in which this invention may be used includes a single element or an ensemble of gain element(s), called active mirrors, and a set of electrical or optical pumping sources. This system may be configured as an oscillator or as a Master Oscillator Power Amplifier (MOPA).
Active mirrors are typically comprised of a thin layer of material that can experience optical gain (i.e. a layer of Nd:YAG, Yb:YAG, or a semiconductor material such as GaAs, in the form of a quantum well or quantum cascade architecture, etc.), which is deposited onto a thin substrate. A highly reflective layer is typically placed in between the active region and the substrate. In general, an optical beam to be amplified impinges this structure, is amplified as it passes through the amplifying layer, reflects from the internal thin reflective layer, and is amplified again as it re-passes through the amplifying layer before emerging from the structure. The active layer can be pumped either optically or electrically depending on the nature of the active layer.
This patent is directed to optically pumped structures. For electrically pumped structures, please see the patent application referred to above. In general, the transverse active region of these active mirrors is limited in scale size due to undesirable parasitic modes that can deplete the stored optical energy (or gain); therefore, they compete for available optical gain with the desired laser mode that is in a direction approximately orthogonal to the transverse dimension. An example of a parasitic mode is an optical beam that propagates, or traverses, in the plane of the active mirror, thereby depleting the optical stored energy.
Conventional laser systems use several approaches to limit or circumvent parasitics and ASE loss mechanisms. One approach involves limiting the physical size of the gain medium, or the transverse spatial extent of the uniform pump beam. The article “Scalable Concept For Diode-Pumped High-Power Solid-State Lasers”, by A. Giesen et al., published in Applied Physics B 58, 365-372,Springer-Verlag (1994), describes a quasi-three-level laser gain media element that employs thin disk stages attached to coolers. In this application, the size of the surface area of the disk has to be limited due to parasitics, while the thickness is limited by thermal considerations. These limitations in size dictate a reduction in the size of the usable surface area of the gain medium, which results in a lower number of the usable pump photons. See also “Scalable High Power Optically Pumped GaAs Laser”, by Le, Di Cecca and Mooadian, published in Applied Physics Lett.,Vol. 58,No. 18, 1967-1969,American Institute of Physics (1991). The technology disclosed thereby avoids the limitation of the physical size of the gain medium by partitioning the gain medium.
A second approach for circumventing undesirable transverse losses involves physically sectioning or otherwise modifying a large-size gain medium into a number of smaller discrete gain cells, as is described in U.S. Pat. No. 4,757,268 issued in 1988 to Abrams et al. As an example of physically sectioning a large-size gain medium into a number of smaller discrete gain cells, a large transverse area gain medium, such as Nd:YAG, is longitudinally sectioned or sliced into a number of small segments. In addition, loss elements (e.g. absorbing slabs) may be placed between the gain medium elements to avoid transverse parasitics of the package. Further, this technique also requires coherent combining of the discrete amplifying stages to realize optimal far-field performance, which is usually accomplished via adaptive optics or via nonlinear optical phase conjugation. Unlike this second approach, the invention disclosed herein can be realized using a monolithic structure, as opposed to the discrete gain elements. This results in high optical quality across the active mirror, and, since the entire structure is formed on a common substrate, the need to coherently combine the discrete amplifying states can be avoided.
In a third approach to reducing ASE and parasitic oscillation modes, a large-area wafer with a Multiple Quantum Well (MQW) epilayer, serving as the gain medium, is processed during growth to generate discrete gain regions that can yield gain under optical pumping, while other regions cannot, even in the presence of the pump beams. However, this procedure requires additional processing steps during epilayer growth, which adds cost and complexity to the system, while the invention disclosed herein utilizes conventional photolithographic processing techniques. See U.S. Pat. No. 4,249,141,“Laser Systems Using Pentaphosphate Active Mediums”, D. C. Brown, J. Wilson, and assigned to University of Rochester.
A fourth approach for addressing the ASE problem is to employ “optical partitioning” of a single large-area active medium by using an optical pump source with a mask, or other diffractive element, to realize fragmented gain regions, as described in U.S. Pat. No. 5,926,494 issued in 1999 to Pepper. The invention disclosed herein does not require the additional optical elements to map a single large-area optical beam into a prescribed optical pattern for the partitioned pumping, and is not restricted to optically pumped devices.
The Prior Art also Includes:
(1) U.S. Pat. No. 4,757,268,“Energy Scalable Laser Amplifier”, by Abrams, et al. and assigned to Hughes Aircraft Company. While this patent describes an array of individual laser gain elements, it does not suggest a monolithic structure.
(2) U.S. Pat. No. 5,926,494,“Laser Systems with Improved Performance and Reduced Parasitics and Method”, D. M. Pepper, and assigned to Hughes Electronics Corporation. This patent describes a method of reducing the parasitics via a spatially inhomogeneous optical pump beam, but does not suggest a physical change in the gain medium.
(3) A. Giesen et al., “Scalable Concept For Diode-Pumped High-Power Solid-State Lasers”, Applied Physics B 58,pp. 365-372,Springer-Verlag(1994). This reference describes a quasi-three-level laser gain media element that employs thin disk stages attached to coolers, which must be limited in surface area due to parasitics.
(4) H. Le, Di Cecca and Mooradian, “Scalable High Power Optically Pumped GaAs Laser”, Applied Physics Lett.,Vol. 58,No. 18, 1967-1969,American Institute of Physics (1991). This discussion also limits the surface area of the gain medium due to parasitics.